because it was a piece of shit with good games (hard to say this but is true, i have one). It was expensive, underpowered, but with great games, that's why i bought it, but not everybody

So many things. Apart from the marketting, brand name, throwing everything your predecessor worked on down the drain and a lack of interesting games at launch, it just seemed to me that the Wii U lacked identity.

Nintendoland tried to show the uniqueness of the Wii U gamepad yet no game really capitalised on it. There wasn't any vision in it, just simply an idea they hoped would catch on. The games that sold well (MK8, Smash, Mario, Splatoon) were released later in the lifecycle, and the gamepad absolutely added nothing to the experience. On the other hand the Wii motes were integral in core franchises like Zelda, Mario and MK on the Wii. You could tell from year 1 that it became an afterthought.

Ultimately the Wii U had so much going against it, and it dug a hole too deep to get out of. Yet in a way it inspired the Switch and for Nintendo it might've been the stepping stone they needed.

First, Nintendo was going through an arrogant phase in the last few years of the Wii. This lead to games like Mario Galaxy 2 (complete with a DVD telling you how to jump and insulting your intelligence), Metroid Other M, and Skyward Sword (a game about puzzles). The latter is interesting because there was already a Zelda game on the system that sold much better.

Nintendo wanted to make the games they wanted, not the games the market wanted. So Nintendo thought if they use the Wii branding and make their games, they would sell just like Wii Sports, Mario Kart and New Super Mario Bros. This is how 3D Mario became a copy of 2D Mario complete with powerups and a goal flag. Nintendo made Pikmin 3 because Miyamoto wanted Pikmin to be successful. The games and the system were serving Nintendo developers. There was even Wind Waker HD, Aonuma's baby but now with the Wii branding (I should also note the 3DS had a lot of the same problems which promted Nintendo to slash the price. It was games like Mario Kart, Animal Crossing and Pokemon that saved that system).

There was also a lot of disjoint in the library. Nintendo wanted more "mature" games, so they got tons of AAA ports (which is why I gawk when analyst suggest the Switch needs more AAA games. It didn't help the Wii U). Nintendo launched the system with gritty Zombie U but had sugar coated candy land Nintendo Land. You have mature Bayonetta going nude to attack and then DA DA DA Captain Toad's Treasure Tracker. Who was this system appealing too. BTW, Nintendo Land is an another games that hurt the system. Wii Sports wasn't a minigame collection but was a sports compilation. Consumers didn't see it as a collection of small games but a collection of sports games. This is why Nintendo Land never took off. Minigame collections aren't that hot.

Lastly, the Gamepad was never going to work. Look at what Nintendo came up with. Very few of the ideas were actually that great. For example, Star Fox Zero had gryo controls which helped to improve the game, but then you had this gamepad that you had to focus on and you normally wouldn't use it unless you were fighting Star Wolf. In fact, the gamepad took away from that game. The only games that used it well were Splatoon, Mario Maker, and Star Fox Guard. The former is getting a sequel and all they are doing is changing how the map works. So even though Splatoon used it, it wasn't that crucial. Its also clear Breath of the Wild was going to use the gamepad, but was quickly dropped and it doesn't effect the game in the slightest. Nintendo misunderstood why people liked motion controls. Motion controls added to games and made them feel real to the average consumer. Nintendo saw this as "OHHHH, people like integrated hardware/software," and made the gamepad. No one liked the gamepad and Nintendo could never make a breakout hit with it

So, to sum it all up, the issue was the games. Why is the Switch so successful. Because Breath of the Wild is that good. Good games sell systems while bad games don't. Nintendo made bad games that they wanted to make, and the end result was the system flopped. They thought that if they had the Wii branding, everything would be fine. The reality is that consumers are smarter than that.

I remember watching the Nintendo E3 2011 presentation wondering why they were wasting so much time showing some stupid Wii tablet peripheral when they were supposed to be revealing their new console...
Little did I know that was foreshadowing of the Wii U's life...

Back when the Wii U launched it was a multitude of things that made the console fail.

First was messaging, many people (including myself) were confused at least initially what the Wii U actually was. Was it a controller that hooked up to the existing Wii or a new console entirely? Not until you looked at the background in the trailer and seen a differently designed console does it become clear. They focused to much on the controller/tablet itself rather than the console that left confusion. Which caused the casual market to believe it was nothing more than an overpriced peripheral for the existing Wii. 

Nintendo had third party support initially (Batman, Watchdogs, ZombieU, etc.) but it wasn't able to be sustained due to lack of sales and interest as a result that most of the third party games were on the market for a while already. Another issue was that the gamepad itself was a 'one of' and wasn't easy at all to buy another one. It also lacked big hitting games from Nintendo at launch unless you consider NSMB a 'heavy-hitter'. The WiiU also had an issue where it was trying to cater to the casual crowd due to the Wii success. Which ended up not working like the video chat app, etc.

The big differences between the Switch's success so far and the WiiU is that of messaging (you know without a word stated what the initial trailer wanted to detail its a console that can be brought wherever as well as play at home on a tv), games (Nintendo brought out a heavy hitter in the name of Zelda at launch), pricing (its cheaper than most Mobile tablet devices out there), and design.

Last I checked, it was a combination of the name and the marketing. Most people didn't even know it was a new console.

Bad marketing, weak hardware and the gamepad.

I would list a bunch of issues:
- You can't buy a second regular gamepad (tablet). Really, only in Japan. So if your gamepad breaks, you have to buy a second hand gamepad or get a new console. Which kind of people would want this?
- Terrible naming. Like "we" and "you"? I still don't get it. But it's a minor problem.
- Horrible battery life for the tablet.
- Bad controller ergonomics.
- Remote play was bad. You still have to be close to the console, so it's a moot point. Unless you're in Japan, most people have a dedicated gaming TV, so it's useless. It also made games have to work with a single screen, stimulating developers to avoid using the second screen for something relevant.
- Almost zero games had relevant gamepad features. It was a glorified mini-map and inventory management.
- As the gimmick was not interesting, it ended up just being a weak and expensive PS4/X1 alternative with zero 3rd party support.

The last 3 made it DOA. The Switch sells because it has a good "gimmick". Play on the go and seamlessly changes to TV mode. Local multiplayer with a single detachable controller, local MP on the go. So it has features to be different than other consoles on the same price point. Wii U didn't, so it would only have been successful if it, at least, beat the PS4 for a big margin to compensate the lack of games (right now, it would probably have to be around US$$ 150).

I think that the real reason the Wii U failed is that Nintendo got complacent and spoiled by success of the Wii. Actually, the 3DS came very close to failing for a very similar reason (poor price-point, and a gimmick that nobody really cared for) but Nintendo managed to save it with a big price cut in the launch window. The 3DS should have been Nintendo's wake up call, but instead Nintendo continued with the mindset that if they tack on a cheap gimmick with their console they can expect a Wii-like success without any real effort.

The Wii U had virtually no great games at launch and this just reinforces the fact that Nintendo really didn't think that they had to put in any effort to make a console successful. That's what I love about the Switch: it's obvious that Nintendo is throwing everything that they can to make the console successful and it is really paying off. The Wii U actually had some amazing games from Nintendo but most of them came later in the console's life after Nintendo had woken up, but at that point it was too late for the Wii U.

It had no 3rd party games and you couldn't bring the wii u tablet outside the room you're in bad battery life and only kids games there were a few exeptions like bayo and zombiu and the cod ports also you cant get a new wii u tablet